Active Microrheology of Dense Colloidal Suspensions

We investigate the active microrheology of a colloidal suspension using laser tweezers. The experimental system described here is composed of a hard sphere suspension of fluorescent, index-matched poly(methyl methacrylate) particles seeded with a low concentration of index-mismatched melamine probes. The probe particles are held in an optical trap and subjected to a uniform flow, enabling measurements of the suspension microrheology. Additionally, confocal microscopy is used to obtain non-equilibrium microstructural information. An anisotropic pair distribution function, with a dense region at the leading surface of the probe and a wake trailing it, is observed as the P\'{e}clet number increases to much greater than unity. This structural transition gives rise to a shear thinning regime in the measured microviscosity. The results are in qualitative agreement with recent simulation [I. C. Carpen and J. F. Brady, J. Rheol. 49, 1483-1502 (2005)], and demonstrate the non-linear microrheology of colloidal suspensions.